3.6 IsPostBack

The page exposes the IsPostBack property. This is a
read-only Boolean property that indicates if the page or control is
being loaded for the first time, or if it is being loaded in response
to a client postback. Many expensive operations (such as getting data
from a database or populating ListItems) must be performed only the
first time the page or control is loaded. If the page is posted to
the server and then reloaded, there is no need to repeat the
operation. By testing the value of IsPostBack, you can skip the
expensive operation, as in the code snippets in Example 3-1 and Example 3-2.

Example 3-1. Testing for IsPostBack in VB.NET

sub Page_Load(ByVal Sender as Object, _
ByVal e as EventArgs)
if not IsPostBack then
' Do the expensive operations only the
' first time the page is loaded.
end if
end sub

Example 3-2. Testing for IsPostBack in C#

void Page_Load(Object sender, EventArgs e)
{
if (! IsPostBack)
{
// Do the expensive operations only the
// first time the page is loaded.
}
}